Help with Predicting Outcomes

By Craig W. Norman, FCLSA

Why is this? Is it today's soft lens designs or is it the patients? In my March 2010 Prescribing for Presbyopia column, "Topographer Applications for Presbyopia Patient Analysis," I discussed how topographical measurements can help determine pupil size. Another benefit of corneal topography is the ability to determine the location of a patient's visual axis and thus how this will correspond to the optics of a simultaneous hydrogel lens design.

The design concept of centernear and center-distance hydrogels considers the influence of pupil size, lens centration, and lack of translation. The principle of simultaneous vision depends on average pupil size, pupil size changes that are reflective of change in gaze, and convergence and alignment of the lens optics within the pupillary area.

Clinically, we know that it is not uncommon for a soft lens to be decentered from the pupillary (and visual) axis, often inferiorly and laterally with little translation during the change from primary and downward gaze.

Help in Predicting Outcomes

So what does this mean to us? The ideal presbyopic fit will have the center-near optics positioned directly over the visual axis in all gaze positions with the pupil being sufficiently large enough in distance gaze to utilize 70 percent or more of the distance optics. Conversely, in near gaze the pupil should become small enough and have the near portion of the lens centered over the visual axis to provide for the same 70 percent of the optics for reading.

Here's where corneal topography can be an excellent predictor of final fitting results—not for surface measurements, but for a hint on whether the lens optics will align properly on a given eye.

Figure 1 demonstrates a uniform corneal topography with the concentric rings centered over the whole cornea, combined with a 4.5mm pupil. In this case, the pupil is about average size, but most importantly, we can expect that the soft lens fit will be wellcentered, placing the center-near Our goal: correcting presbyopes to provide maximum vision at all distances. Our results: we often don't achieve the desired vision with simultaneous vision soft lens designs. optics in an ideal position within the pupil.

Figure 1. Example of uniform corneal topography.

In Figure 2, the reflective rings are noticeably decentered laterally along with a larger 5.5mm pupil. The concern in this case is that if we attempt to fit a perfectly centered lens, the near optics will not be aligned with the visual axis in reading gaze. In essence, the lens fits great but the visual results are poor. Also, if this patient had a pupil size similar to Figure 1, we would expect diminished reading ability.

Improving Fitting Results

Corneal topography is an ideal screening tool to rule out potentially poor visual results such as in Figure 2. We've found this to be extremely helpful in determining which presbyopic candidates may have the best chance of visual success. CLS

Craig Norman is director of the Contact Lens Section at the South Bend Clinic in South Bend, Indiana. He is a fellow of the Contact Lens Society of America and is an advisor to the GP Lens Institute. He is also a consultant to B&L. You can reach him at craignorm@aol.com.